It is desirable to be able to sense the temperature of a gas in an environment (e.g., a gas turbine engine) subject to vibration, high temperature, dirt and other factors which are generally abusive to scientific instruments. Known temperature sensors include thermocouples, pyrometers and black body temperature sensors. In one type of black body sensor, the tip of a high temperature lightpipe (e.g., a sapphire rod) is coated with a material (e.g., a noble metal) which forms a black body. Alternatively, a noble metal tip may be attached directly to the rod. When heated, the black body material emits radiation which is transmitted through the lightpipe to, for example, an optical fiber, this insulating the optical fiber from the high temperature environment. Examples of this type of temperature sensor are illustrated in U.S. Pat. Nos. 4,576,486, 4,859,079. This type of temperature sensor may not work well at high temperatures where the black body material may separate from the lightpipe. Further, since the lightpipe is not normally shielded, contamination may build up on its surface, degrading its optical transmission characteristics. Finally, mismatches in the thermal coefficient of expansion between the rod and the black body may degrade performance at high temperatures.
Alternative temperature sensors may employ a black body (e.g., a hollow core) which is separated from the optical fiber. Radiation from the black body may be focused on the tip of the collection fiber by, for example, a lens. Temperature sensors of this type are illustrated in U.S. Pat. Nos. 4,737,038 and 3,626,758. The size of this type sensor is limited by the size of the lens. This is a disadvantage because the accuracy of black body radiators of this type increases as the size of the black body decreases. Further, small sensors are advantageous because they are easier to shield and mount.
Pyrometers may use a first lens to collimate radiated energy which is transmitted through an inert gas and focused by a second lens onto, for example, an optical fiber.